The present invention relates to a piezoelectric resonator used for a filter, an oscillator, etc., and more particularly, to a highly stable high-frequency piezoelectric resonator which can be employed in a fundamental thickness-extentional-vibration mode in the VHF and UHF bands.
In general, a piezoelectric resonator is employed in a thickness-extensional-vibration mode of a piezoelectric thin plate, in a high frequency band. The following types of resonators have been known as piezoelectric resonators for a high frequency use:
(1) a piezoelectric resonator formed by polishing a piezoelectric plate (such as a quartz plate or a piezoelectric ceramics plate) into a thin plate and used in a fundamental vibration mode, PA1 (2) an overtone resonator which utilizes a higher-order overtone vibration of a piezoelectric plate, such as a quartz plate or a piezoelectric ceramic plate, and PA1 (3) a piezoelectric thin film resonator formed by a substrate of silicon or quartz and thin films of piezoelectric material such as ZnO, CdS, and AlN and of conductive electrode material formed on the substrate.
In the above mentioned resonator (1), the fundamental resonant frequency can be made higher in an inverse proportion to the thickness of the plate, by making the piezoelectric plate thinner. However, the difficulty in manufacturing is increased as the plate is made thinner. At present, the practical limit of the fundamental resonant frequency is about 50 MHz with a plate thickness 30-40 .mu.m. The above mentioned resonator (2) has only a small electromechanical coupling coefficient due to the use of overtone mode vibration. Hence, a frequency band width sometimes becomes so small that it cannot be provided in a practical use. Moreover, a lower-order vibration having a larger electromechanical coupling coefficient would become a spurious vibration.
The above-mentioned resonator (3) has a large electromechanical coupling coefficient in the high frequency band of several hundreds MHz and can be utilized in a fundamental vibration even in the VHF and UHF band. However, ZnO, CdS, AlN, and other representative piezoelectric materials for the piezoelectric thin film have a large frequency temperature coefficient. Therefore, it is impossible to produce a piezoelectric resonator having a high temperature stability.
As a countermeasure for improving the temperature stability of the piezoelectric thin film resonator, it has been proposed to reduce an absolute value of a frequency temperature coefficient of a piezoelectric resonator, as a whole, by combining a piezoelectric material having a negative frequency temperature coefficient and a material having a positive frequency temperature coefficient. In detail, the article entitled "ZnO/SiO.sub.2 -Diaphragm Composite Resonator On A Silicon Wafer" appearing in "Electronics Letters" Vol. 17, No. 14, p.p. 507-509, disclosed a piezoelectric thin film resonator having a SiO.sub.2 film of a positive coefficient formed on a surface portion of a silicon substrate. A lower electrode film is formed on the SiO.sub.2 film. A ZnO piezoelectric thin film of a negative coefficient is deposited on the lower electrode film, and an upper electrode film is formed on the top. The portion of the silicon substrate exactly under the vibrating location is finally etched away. However, this structure still has a drawback, as will be explained later in detail with reference to the drawings. The thickness of the SiO.sub.2 film becomes considerably larger as compared with the ZnO film for giving a zero temperature coefficient. Hence, the ZnO film is considerably deviated from a position of symmetry with respect to a vibrational nodal point of a fundamental thickness-extensional-vibration mode. The 2-nd order, 4-th, order, and the other even-number order harmonic overtones are strongly excited, as spurious modes, in addition to odd-number order hamonic overtones.
As a trial for suppressing the above-mentioned even-number order harmonic overtones, a thin film composite resonator is proposed. An additional SiO.sub.2 thin film is provided on the upper electrode. In other words, a pair of SiO.sub.2 thin films are provided on opposite sides of the upper and lower electrodes of the ZnO piezoelectric film, the SiO.sub.2 films being symmetrically positioned with respect to the ZnO thin film. With this structure, the central portion of the ZnO piezoelectric thin film serves as a nodal point of vibration. The suprious effect due to the even number order harmonic overtones can be suppressed because electric charge is offset within the piezoelectric thin film. However, with regard to the film thickness ratio for obtaining a zero frequency temperature coefficient, the ZnO piezoelectric thin film (relative to the total film thickness at the vibrating location in this composite resonator) becomes thinner than the SiO.sub.2 thin films. Thus, a good energy trapping becomes impossible. Accordingly, it is difficult to obtain an excellent resonance response in a resonator having a zero temperature coefficient.